Conventional concrete and wooden transit platforms have a durability problem due to degradation by environmental chemicals such as, salt, urea, acid rain, oils and greases as well as stray electrical currents. This necessitates regular maintenance and periodic replacement of the platforms at considerable cost to transit authorities. Replacement is further complicated with trains going by the platform every few minutes. Steel and concrete are also susceptible to corrosive elements, such as water, salt water and agents present in the environment such as acid rain, road salts, chemicals, oxygen and the like. Environmental exposure of concrete structures leads to pitting and spalling in concrete and thereby results in severe cracking and a significant decrease in strength in the concrete structure. Steel is likewise susceptible to corrosion, such as rust, by chemical attack. The rusting of steel weakens the steel, transferring tensile load to the concrete, thereby cracking the structure. The rusting of steel in stand alone applications requires ongoing maintenance, and after a period of time corrosion can result in failure of the structure. The planned life of steel structures is likewise reduced by rust. Wood, like concrete and steel, is also susceptible to environmental attack, especially rot from weather and termites. In such environments, wood encounters a drastic reduction in strength which compromises the integrity of the structure. Moreover, wood undergoes accelerated deterioration in structures in marine environments.
Concrete transit platforms are typically constructed with the concrete poured in situ as well as using some preformed components pre-cast into structural components such as supports and transported to the site of the construction. Constructing such concrete structures in situ requires hauling building materials and heavy equipment and pouring and casting the components on site. This process of construction involves a long construction time and is generally costly, time consuming, subject to delay due to weather and environmental conditions and the requirement not to disrupt the schedule of trains unduly.
On the other hand, pre-cast concrete structural components are extremely heavy and bulky. Therefore, they are also typically costly and difficult to transport to the site of construction due in part to their bulkiness and heavy weight. Although construction time is shortened as compared to poured in situ, extensive time, with resulting delays, is still a factor. Construction with such pre-cast forms is particularly difficult, if not impossible, in areas with difficult access or where the working area is severely restricted due to adjoining tracks, buildings or platforms. There is a need for a light weight structure to facilitate installation in areas which have difficult access and working area. In addition a lightweight structure could eliminate the costly concrete foundations and steel support systems necessary to support conventional concrete platforms.
There have been solutions proposed for preventing deterioration of steel and concrete bridge and roadway decks. For example U.S. Pat. No. 5,901,396 discloses the use of an aluminum bridge deck to provide light weight and durability. In addressing the limitations of existing concrete, wood and steel structures, some fiber reinforced polymer composite materials have been explored for use in constructing parts of bridges including foot traffic bridges, piers, and decks and hulls of some small vessels. Fiber reinforced polymers have been investigated for incorporation into foot bridges and some other structural uses such as houses, catwalks, and skyscraper towers. These composite materials have been utilized in conjunction with, and as an alternative to, steel, wood or concrete due to their high strength, light weight and highly corrosion resistant properties. However, construction of load bearing applications built with polymer matrix composite materials have not been widely implemented due to extremely high costs of materials, high assembly costs and uncertain performance, including doubts about long term durability and maintenance. As cost is significant in the public transit industry, such materials have not been considered feasible alternatives for many load bearing traffic designs. For example, high performance composites made with relatively expensive carbon fibers have frequently been eliminated by cost considerations.
U.S. Pat. No. 5,794,402 is directed to a polymer matrix composite modular load bearing deck as a part of a modular structural section for a highway bridge deck. The load bearing deck is formed from a plurality of sandwich panels, each panel having a flat upper surface, a lower surface and a core. The core includes a plurality of trapezoidal, substantially hollow, elongated core members positioned between the upper surface and the lower surface. Each core member has side walls positioned generally adjacent to a side wall of an adjacent core member and are joined together by fasteners, such as bolts and screws, or by adhesives. The assembly time required to fasten the deck together renders the cost prohibitive and impractical for use in a transit platform.
In public transit facilities, such as subway stations and railway stations, there is also a requirement for pedestrians to be able to safely navigate the platform. There is a need for pedestrians to get good traction on the platform to prevent slips and falls in particular on outdoor platforms that can be subject to wind, rain and snow conditions. In addition it is important for pedestrians to be able to detect the location of platform edges so that the pedestrian does not accidentally walk off the edge of the platform. The need for making platform edges detectable is of course particularly acute in attempting to make such facilities accessible and safe for blind or visually impaired persons.
In the 1980's a series of studies were undertaken in the United States to improve the design of buildings and transportation facilities to improve the mobility of the visually impaired. These studies culminated in recommendations on making potential hazards detectable to the visually impaired either by use of the long cane or underfoot.
Americans with Disabilities Act (ADA): Accessibility Guidelines for Buildings and Facilities set the requirements for the use of detectable warnings on inter alia transit platforms to warn visually impaired persons of hazards. The Guidelines require that detectable warnings shall consist of raised truncated domes of prescribed diameter, height and center-to-center spacing and shall contrast visually with adjoining surfaces. Detectable warnings used on interior surfaces are required to differ from adjoining surfaces in resiliency or sound-on-cane contact. Various tactile tiles having raised truncated domes in compliance with the ADA Guidelines or the equivalent have been developed such as those shown in U.S. Pat. Nos. 4,715,743 and 5,303,669. Other tactile surfaces have been proposed such as the rubber on concrete composite tile illustrated in Netherlands Patent 8600855.
U.S. Pat. No. 5,303,669 describes a detectable tactile tile that is intended to be installed in concrete or the like. The tiles are illustrated as square with depending flanges projecting downward from the edge of the tile. The flanges have holes through them to assist in anchoring the tile in freshly poured concrete. The holes in the flanges around the perimeter of the tiles permit air to flow out from under the tiles when they are pressed into the concrete. However it is virtually impossible to remove all of the air and there is typically an air space between the bottom surface of the tile and the top of the cured concrete. When baggage carts, money carts with small wheels or heavy mechanical equipment either for cleaning, snow removal etc. passes over the tiles, there may be a tendency for the tiles to crack under the weight of the equipment, due to the air space between tiles and the concrete surface.
U.S. Pat. No. 5,775,835 provides a tactile tile for embedment in fresh concrete on a platform or walking surface. By anchoring the tiles with the concrete through holes in depending flanges the need for adhesives or mechanical fasteners which are labor intensive to install are eliminated or reduced. The bottom surface of the tile is provided with a series of projections. As the tile is being pushed into the concrete the projections assist in having the concrete flow underneath the tile and as the concrete cures and shrinks slightly the projections remain in contact with the cured surface of the concrete so that the tile is fully supported across its surface. During snow removal or cleaning, the tile will then support the weight of any heavy mechanical equipment and eliminate cracking of the tiles and their necessary replacement. As the fresh concrete cures, an air space forms between the bottom surface and the surface of the cured concrete. This air space prevents the load from equipment moved over the tiles from being transferred to the platform surface resulting in potential damage to the tiles. By incorporating the projections into the bottom surface the loads can be transferred to the platform or walkway surface through the conical standoffs. However the airspace between the concrete surface and the bottom surface is not eliminated resulting in a hollow sound when struck by the cane of a visually impaired person. This distinct sound-on-cane contact between the tiles and the adjoining concrete surface permits the tiles to be used indoors in compliance with the ADA Guidelines. Where the tiles are bonded by an adhesive or mechanically fastened directly to the concrete surface it may not be possible to get a distinctive sound-on-cane contact with a hard material of manufacture such as ceramic, glass reinforced thermosetting resin or vitrified polymer composite and softer resilient rubber or vinyl tiles must be used. In addition use of the projections increases the surface area of the tile that is in contact with the cured concrete which helps resist movement due to thermal expansion etc.
In conventional systems there is also a problem with drainage. Corrosive elements can penetrate past poorly installed or worn sealant joints leading to the deterioration of the steel support structure and concrete foundation.